1. Field of the Invention
The present invention relates to gas nozzles. In particular, the present invention relates to an improved gas nozzle for use to support a continuous web of moving material such as paper, film, textiles, etc.
2. Description of the Prior Art
Web dryers have been used in the manufacture of paper and the like and in the printing and coating of webs of paper, synthetic materials, films, etc. A gas or vapor, such as steam or air, is supplied from one or more airbars and is used to float a continuous web of material such as paper, film, textiles, cording, steel, etc. By heating or cooling the air relative to the continuous web material, heat can be transferred to or from the material by forced convection, thereby aiding in effective temperature changes, evaporating solvents from the web, curing material added to the web, etc.
Prior attempts to fulfill these objectives have one feature in common: the air jet exiting the airbar is formed by converging or parallel passages which accelerate and smooth the flow. Any discontinuities in the passageway, such as those introduced by structural supports (pins), welds, or hole boundaries (in the case of jets formed by discrete holes instead of continuous slots), leave a wake in the jet stream which causes cross-web variations in air flow heat transfer.
Most of the prior devices have been designed to provide a specific heat transfer pattern. For example, U.S. Pat. No. 3,549,070 to Frost et al. uses two jets of air impinging on the web, causing two peaks of high heat transfer. U.S. Pat. No. 3,587,177 to Overly et al. uses the Coanda effect (discussed in U.S. Pat. No. 2,052,869 to Coanda) to create a flow parallel to the web. This results in a moderate, relatively even heat transfer in the web direction.
Although these prior art methods are adequate for a specific process, a web dryer may be needed for a variety of processes, each of which has specific heat transfer requirements. Even within one process, such as the drying of a clay coating on paper, the early and late stages of the drying cycle may tolerate high heat and mass transfer rates, while an intermediate stage may require extremely even and moderate to low heat and mass transfer rates.
It is desirable from an engineering and manufacturing point of view to have a variety of airbar designs, each of which is made nearly the same, yet each of which can be used to obtain different heat transfer patterns and web handling characteristics. These various characteristics may be required due to the differences in web structural characteristics, such as weight, strength, stiffness, thickness, etc. and in web tension control levels.